The present invention relates to a standard for use in an optical line width measuring machine to monitor its calibration.
In VLSI and related technology it is becoming increasingly important to be able to measure line widths to an accuracy down to a micron as device geometries shrink. The present method of making such measurements is to use an optical line width measuring machine supplied by one of several manufacturers. Such machines are capable of measurements to the nearest micron or less but must be periodically monitored to detect drift. It is common practice to establish a standard for checking calibration which consists of a layer of photoresist deposited over a slice of silicon substrate with the photoresist patterned to have a slot. With light shining down onto the photoresist a phototube measuring reflected light indicates two spaced apart dips in intensity corresponding to the rounded edges of the photoresist slot. By curve fitting the obsrved intensity spectrum using software normally supplied by the manufacturere of the machine it is possible to extract a width measurement for selected points on each intensity trough. The latter value should remain unchanged over a period of time.
Thus, it is extremely important that the photoresist standard not change over a period of time. However, standard photoresist is subject to shrinkage or aging and is unreliable as a standard.
Accordingly, it is an object of the invention to provide an improved standard for use in checking the calibration of optical line width measuring machines.